The Color of White: Is there a "preferred" color
temperature for the exhibition of works of art?

by Steven Weintraub

What is the color of white light? This may seem like an odd
question. After all, white light is white -- the combination of all
the colors of the rainbow. And yet, depending on the context, white
light does not always appear to our eye as white. When an observer
stands in an atrium filled with "white" daylight and looks at a
gallery illuminated with "white" tungsten-halogen lamps, the
artificial light appears yellow or unnaturally warm. When the
visitor stands in an artificially lighted space where there is no
reference to daylight, the light appears "white" and a small spot of
natural light will appear blue or unnaturally cool compared to the
primary tungsten-halogen light source. So, which light is "white"?
The answer is -- both, even though they differ in the blue-to-red
proportion of visible energy. In fact, there is a wide variety of
"whites," ranging from cool to warm, determined by the varying
proportion of blue to red.

When we compare different types of light sources such as
incandescent lamps, tungsten-halogen lamps, various fluorescent
lamps, and daylight during different times of the day, each source
appears to transmit a different quality of "white" light, especially
when these sources are seen side-by-side. The difference in white is
defined by the "color temperature" of the source. The concept of
color temperature is based on the observation that a substance
heated to high temperatures emits visible radiation in a broad
spectrum. At 2000° K, the emitted light looks orange-yellow,
based on the high proportion of long "warm" wave lengths. As the
temperature increases to 20,000° K, it appears blue, based on
the high proportion of short "cool" wavelengths. Between these two
extremes, the light appears to be "white" rather than yellow or
blue, although the "white" light will range from warm to cool,
depending on such factors as intensity and context. Since "white"
light covers a broad range of color temperatures, how is our
perception of a work of art affected by the choice of illumination?
Is there an ideal white light, i.e. is there a preferred color
temperature for viewing works of art?

The traditional response to the question of the optimum light
source for viewing art has been to use the same type of light in
which the object was either created or intended to be seen by the
artist. Prior to the use of modern high color temperature sources
like fluorescent lamps, this would have been either natural light
(preferably northern light which has a very high color temperature),
or sources such as a candle, gas light or an incandescent lamp which
have very low color temperatures. Based on the fact that many
artists preferred to work in daylight, it has been assumed that
daylight is the best illumination source for viewing art. Many
museums have spent enormous sums of money on systems that
incorporate high color temperature natural light, especially for
galleries where oil paintings are exhibited. Is this assumption
about using high color temperature natural light valid? According to
research published over half a century ago, the answer is an
unequivocal "no".

In 1941, a Dutch researcher, A.A. Kruithof ("Tubular Luminescence
Lamps for General Illumination," Philips Technical Review,
vol.6, 65-96, 1941), published a graph (see illustration)
summarizing the relationship between color temperature, intensity,
and the "pleasant" quality of an illumination source. According to
the Kruithof curve, an observer prefers lower color temperature
lighting when the light level is lower, and prefers a higher color
temperature when the light level is higher. In essence, Kruithof
provided a quantitative basis for describing a phenomenon that we
all experience. For example, a space uniformly illuminated at 20
footcandles by daylight at a color temperature above 6000° K
appears gloomy and overcast, whereas the same space illuminated at
the same 20 footcandles with tungsten-halogen lamps at 3000° K
appears pleasant and comfortable. The conclusion drawn from
Kruithof's curve is that our color-temperature preference changes
based on the intensity of the light within a space.

When an artist paints outside or by
an open window, the color temperature is very high and so is the
intensity of the light. When the painting is exhibited inside a
museum at 20-30 footcandles, it is illuminated at a much lower level
of intensity. According to the Kruithof curve, if the painting is
illuminated at the same color temperature under which it was created
(for example, 10,000° K), but at a much lower light level (for
example, at 20 footcandles), the condition of display would be
unpleasant. Therefore, it is not desirable to illuminate a painting
at the same color temperature under which it was created if it will
be exhibited at a much lower level of light intensity. The
appropriate color temperature must be selected based on the
intensity of the ambient illumination.

Kruithof's curve describes the general experience of light as
pleasant or unpleasant. This quality deals with general or ambient
conditions. Although quality of ambiance is important, Kruithof's
work provides no information on how color temperature affects the
observer's perception of specific colors and color relationships. A
review of the technological literature does not provide adequate
answers to questions about the impact of color temperature on visual
perception.

The relationship of color temperature to color perception is not
obvious due to the remarkable ability of human vision to compensate
for wide variations in the spectral distribution of light sources.
This ability, referred to as "color constancy", is similar to the
"white balance" adjustment on a video camera which takes into
account the color temperature of the light source. Color constancy
explains our ability to perceive colors in the same way under a wide
variety of viewing conditions. When we look outside through a green
tinted window, the scene appears natural because the brain
compensates for the green tint and normalizes the view. However,
when we see the same scene through a partially opened window where
the eye compares the two views, we become aware of the tinted glass.
The colors seen through the green tinted portion appear to be
distorted because the brain normalizes colors and color
relationships based on the untinted view and recognizes the
distortions seen through the tinted glass. Although color constancy
reduces the impact that a colored filter or a shift in color
temperature has on color perception, does it fully compensate for
such differences? The question remains--how important is color
temperature in viewing a work of art?

According to recent research by myself and colleagues in the
fields of visual psychology and museum lighting, color temperature
plays a very important role in how one views a work of art. A
variety of experiments, conducted under controlled laboratory
conditions and in museum galleries, are providing a new
understanding of the interaction of color temperature and visual
perception.

Studies were carried out utilizing an ingenious combination of
equipment developed by Kevin McGuire, Tailored Lighting Inc., which
allowed me to evaluate the effect of small incremental changes in
color temperature. The test equipment utilized conventional
3000° K tungsten halogen MR-16 lamps, and special MR-16 lamps
rated at 4700° K developed by McGuire (These lamps are
commercially available under the brand name "SoLux" and will be
described in greater detail in a future WAAC article). By altering
the voltage in small steps with a programmed controller, the
intensity and color temperature of each lamp is modified. When
mixed, the combined output of both lamps produces a full-spectrum
source at intermediate color temperatures between 3000° K and
4700° K with a minimal change in intensity.

A wide variety of color reproductions of paintings were examined
under different color temperatures. The results were surprising. All
the observers in the initial experiments preferred a similar, narrow
color temperature range, regardless of the palette or subject of the
painting. Further confirmation of these unexpected results took
place in museum field tests. The first test took place in 1995 at
the National Gallery of Art (NGA), during the special exhibition of
paintings by Vermeer. Gordon Anson, Chief Lighting Designer at the
NGA and an associate in much of my lighting research, arranged
access to the Vermeer Exhibition and adjacent galleries containing
the permanent collection of 19th century European paintings. Jay
Kreuger, a painting conservator at the NGA, also participated in the
study.

Each painting was examined at 20 footcandles utilizing a variety
of color temperatures. We all agreed that there was a dramatic
change in the appearance of all paintings as the color temperature
varied. We all preferred the same color temperature for individual
paintings within a narrow tolerance of about 200° K. Regardless
of whether they were Impressionist paintings of bright, cool outdoor
scenes, or dark, warm Dutch interiors, the color temperature
preference was within 200° K for all paintings, The two
exceptions, a painting by Turner and The Lace Maker by Vermeer, were
unusual because of the yellowed condition of the varnish. In these
two instances, we all preferred a slightly higher color temperature
(300° K higher) which diminished the impact of the yellow
varnish.

Subsequent tests in other museums yielded the same results. The
most dramatic test took place at the Memorial Art Gallery of the
University of Rochester, Rochester, N.Y.. The late 19th century
French Painting gallery was permanently illuminated with 3500° K
SoLux lamps under the supervision of Candace Adelson, Curator of
European Art, who also played a key role in earlier studies. A
visitor comment book was provided, along with a brief statement
regarding the experimental nature of the gallery lighting. Visitors
were asked to comment on the quality of the lighting compared to
their memory of how the gallery had been illuminated, and compared
to adjacent galleries, illuminated with incandescent (2700° K)
and tungsten halogen (3000° K) lamps at the same level of
intensity. The book was filled with observations about the increase
in the saturation of colors, the greater sense of depth within the
paintings, and the improved brightness and clarity of the paintings
and of the gallery space in general. These comments were similar to
those expressed by observers in other experiments when asked about
how the paintings alter with changes in color temperature.

The fact that most observers chose the same preferred color
temperature within a narrow range is further evidence that the
choice of color temperature involves more than an arbitrary
aesthetic preference. It is based on a fundamental property of human
vision. To further understand the preference for a specific color
temperature, additional studies were undertaken in a non-art
context. A white reflective surface was illuminated at a fixed
intensity as the color temperature was increased and decreased in
small increments between 3000° K and 4700° K. Observers were
asked to describe the light as warm, cool or intermediate. For a
surface illuminated at 20 foot candles, a value around 3700° K
was chosen as the intermediate value, measured with a Minolta
photographic color temperature meter (Model II). At 20 foot candles,
3700° K appears as an achromatic white light compared to higher
or lower color temperature sources. Coincidentally, the choice of
3700° K was the preferred color temperature chosen on aesthetic
grounds when looking at paintings. This suggests that the aesthetic
preference for a specific color temperature derives from a
fundamental characteristic of human color perception.

These color temperature investigations are still at an early
stage. Through the support of a two-year research grant from the
National Center for Preservation Technology and Training further
research will be conducted by myself and specialists in color vision
research at the City University of New York. There are many
questions that require more careful study. For example, the
preferred color temperature readings taken with the Minolta meter
must be reconfirmed with more accurately calibrated instruments.
Another variable that requires further study is the relationship
between the preferred color temperature and intensity. Based on the
Kruithof curve and experiments in progress, it is probable that the
preferred color temperature will shift, based on the intensity of
the illumination source. Beyond the theoretical questions, there is
a wide range of issues involving practical application that must be
understood in order to utilize the results of these studies. A
number of these application issues will be discussed in a subsequent
WAAC Newsletter.